(1) Field of the Invention
The present invention relates to a method and an apparatus for drying granular objects in which the granular objects are dried by being exposed to drying air, and more particularly to a method and an apparatus for drying granular objects which utilize a structure for elevating the temperature of granular objects in advance to their exposure to the drying air.
(2) Description of the Related Art
As a conventional method for drying granular objects by exposing them to drying air, a common method was one in which a high temperature drying air was produced by a burner and a fan and the granular objects were repeatedly exposed to such drying air. However, for purposes of reducing the drying time and of preventing the quality change of the granular objects caused during the drying, a method involving pre-heating process of the granular objects, that is, a method with which the internal temperature of the granular objects is raised in advance before they are exposed to the drying air, has attracted attention as an improved method. One example of a drying apparatus which utilizes far infrared radiation for purposes of the pre-heating is found in Japanese Patent Publication No. 2789279.
The drying method and apparatus equipped with the far infrared radiation means disclosed in the above patent publication seemingly has achieved as far as the above objects are concerned. However, since the structure is such that the far infrared radiation generator should be installed within the drying chamber, the size of the apparatus inevitably became larger by the installation space of the far infrared radiation generator, and thus the apparatus itself unavoidably became large-sized. That is, basically, in order to obtain the effects of radiant heat by providing the far infrared radiation generator within the drying apparatus, it is obvious that a correspondingly large space is required. Since the recent trend of the drying apparatus has been to make its size small and the improvement has been underway by reducing the drying chamber for exposing drying air to grains, the introduction of a far infrared radiation generator is against the trend of reducing the entire size of the drying apparatus.
Conventional methods with which granular objects are pre-heated without utilizing a far infrared radiation generator have been disclosed in Japanese Patent Application Kokai Publication No. Sho 58-187779, Japanese Patent Application Kokoku Publication No. Sho 60-8434, Japanese Patent Application Kokai Publication No. Sho 62-9174, etc. The technique disclosed in each of Patent Application Kokai Publication No. Sho 58-187779 and Patent Application Kokoku Publication No. Sho 60-8434 relates to an arrangement in which a heated air path and a drying chamber are provided with separate heat sources, respectively. Because of this arrangement, these separate heat sources are required to be separately controlled. With use of a plurality of heat sources and need of controlling them separately, the apparatus will inevitably be costly.
Further, the technique disclosed in Patent Application Kokai Publication No. Sho 62-9174 relates to a system in which the heated air produced by a burner and sucked by an air exhausting means is supplied directly to a heated air path chamber thus enabling to pre-heat the grains, and the heated air is supplied from the heated air path chamber to a drying chamber through a heated air guide path. In this system, not only is it made possible to produce both the heated air and the drying air by one heat source (one burner), but also it is made possible to pre-heat the grains immediately below a tank chamber, so that an advantage in this system over the above described drying apparatus, of U.S. Pat. No. 2,789,279 is that, unlike said apparatus, the system does not become large-sized.
However, as to the temperature control in the above system, the drying air temperature (approximately 40.degree. C.) in the drying chamber at which grains are dried should be made as a reference temperature and, since the heated air path chamber communicates to the drying chamber simply through the heated air guide path, even by taking into account the fact that the temperature of the heated air drops due to a heat loss to occur at the heated air guide path, the temperature of the heated air never drops to the extent that it is suitable for drying the granular objects and, thus, it is necessary that the temperature of the heated air itself generated by the heat source of the burner must be substantially lowered. As a result, it will be impossible to raise the temperature of the air at the heated air path chamber, which is to raise the temperature of grains by being in contact with the grains, to the extent sufficient to contribute to the pre-heating.
It has become evident that, in drying granular objects, it is essential, for safely and speedily drying the granular objects, to pre-heat the granular objects and to maintain the predetermined temperature of the granular objects themselves prior to the exposure to drying air and, toward this end, various technical researches and developments have been made but so far without success. The object of the present invention is to provide a drying method and an apparatus which enable to meet the demand for a small-sized drying apparatus, and to achieving a desired drying air temperature with a high pre-heated temperature being maintained, thereby enabling the provision and manufacture of low cost drying method and apparatus with which it is possible to conduct the drying of granular objects safely and speedily.